Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene [1]. Different mutations have a range of effects on the levels of CFTR protein and its proper functioning in epithelial transport of Cl- and HCO3- [2,3]. The severity of the pancreatic phenotype in human CF is well correlated with the extent of impaired CFTR function caused by specific mutations. Loss of CFTR function results in destruction of the exocrine tissue and eventual pancreatic insufficiency. On the other hand, the effects of CF on organs including the airways and intestines is less well correlated with specific CFTR mutations and their effects on CFTR protein function [4-8]. This indicates that other genes are likely to be important as modifiers of the CF phenotype.